In photolithography techniques features are usually printed by passing light through a “reticle”, which acts as a mask and is typically formed from glass printed with chrome patterns. In order to print lines the reticle is provided with slits which allow the light to pass through onto the photoresist (a photosensitive layer used to coat silicon in photolithography techniques). The exposed areas of the resist are removed by applying a developer, and the wafer is then etched by applying a suitable chemical.
In the lithographic stages of an IC (Integrated Circuit) or MEMS (Micro Electrical Mechanical Systems) process, light is transmitted through a mask (sometimes called a reticle) to form patterns on a wafer by means of photoresist and its developer. A common method of defining patterns on a wafer is by means of a step-and-repeat system which involves use of a reduction lens and a mechanical stage. The mechanical stage supports the wafer, and is arranged to move it accurately over given distances. Maximum field sizes on the wafer of reduction lenses (usually 5× reduction) are of the order of 20 mm square so that by means of accurately moving the wafer stage a series of many identical fields can be defined. The size of each circuit component exposed by the reticle at any given time may be much smaller than the field size, which is simply the maximum size that can be exposed at any one time. Contemporary stage technology requires that the registration of patterns to those on an underlying layer can be achieved by global alignment only whereby the whole of the wafer is aligned at 2 or 3 points to marks on the mask. Thereafter, because stage accuracy is very good, of the order of 100 nm over a distance of 22 mm, the series of fields is defined, after the global alignment, by “blind” stepping the stage, that is by moving the stage without further alignment.
Sometimes circuits require an area greater than the maximum field size available in a step-and-repeat system for correct operation. This situation provides increased demands for lithography but can be achieved by means of placing multiple elements on the mask, some of which are used more than once, and then “stitching” together the elements formed by each of the exposures. There is of course no literal “stitching”, but the word “stitching” is used to refer to the accurate positioning, or butting, of one exposure next to another.
The invention seeks to address the problem of ensuring accurate “stitching” between different adjacent exposed areas.